The present invention relates to an apparatus for producing low resistivity tungsten thin films by plasma enhanced chemical vapor deposition (PECVD) technique and more particularly, to a temperature measuring system inside the PECVD reactor, which enables one to measure exact surface temperature of the silicon substrate; one of the major factors to produce low resistivity tungsten thin films.
At present, in general, deposition techniques of tungsten films are divided broadly into two main categories; physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques. The former technique, for instance, utilizes sputtering or electron beam melting of the high purity target and the latter utilizes the thermal decomposition of a WF.sub.6 -H.sub.2 gas system at low pressure, which is the low pressure chemical vapor deposition (LPCVD) technique.
When the PVD technique is used to produce thin films of tungsten, the problems such as high probability of defect formation due to the radiation damage and poor step coverage arise. Moreover resistivity of the deposited tungsten thin films is higher than 50 micro-ohms-cm.
The use of the LPCVD technique can obtain high quality tungsten thin films free of the problems associated with PVD technique listed above, which is a distinguishing feature of the LPCVD technique. However the conventional LPCVD technique has a difficulty in the initiation of tungsten nucleation on substrates other than silicon such as insulator or compound semiconductor material. Therefore when the tungsten film is needed to be deposited on any substrate type, LPCVD techniques cannot meet the requirements. [Please refer to W. T. Stacy, E. K. Broadbent, M. H. Norcott, J. Electrochem. Soc., 132, 444 (1985)].
Plasma enhanced CVD (PECVD), one of the newly developed CVD techniques, allows one to deposit tungsten thin films onto any substrate and produce conformal step coverage better than that of LPCVD films, which is about 10 micro-ohms-cm, and it limits PECVD technique to be applied to the real device fabrication. [Please refer to C. C. Tang, D. W. Hess, Appl. Phys. Lett., 45,633 (1984)].
In order to produce high quality PECVD tungsten thin films with low resistivity, it is important to measure and control the deposition temperature accurately, since it is the major factor influencing the quality of the deposited films.
Since the surface temperature of the substrate depends on the parameters such as the temperature of the hot plate and total pressure in plasma ambience, the conventional PECVD system measures the temperature of the hot plate which is a certain distance apart from the substrate as in FIG. 1 instead of measuring temperature of the substrate directly. Consequently, the deposition temperature is not determined by the actual substrate temperature and can lead to quite erroneous results.
In order words, referring to FIG. 1, in the conventional PECVD system, where diffuser/electrode 2 is placed on the upper part of a reactor 1 and a hot plate 4 heated by a heating wire 3 on the lower part of the reactor 1, temperature of the hot plate 4 with the silicon wafer 5 Placed on the top is measured by the thermocouple T, so that there is a great difference between the actual surface temperature of the substrate and that of the hot plate 4.
Unlike the conventional PECVD system described above, there is a way to measure temperature of the wafer directly with the optical pyrometer, but the emissivity changes as the tungsten deposition progresses and also the plasma emission effects the reading of the pyrometer. In result, it is impossible to measure the exact temperature of the substrate. [Please refer to J. E. J. Schmitz, J. L. G. Suijker, M. J. Buiting, Tungsten and Other Refractory Metals for VLSI Applications IV, eds. R. S. Blewer and McConica, Materials Research Soc., Pittsburgh, 211 (1989)].
On the other hand, there is another way to measure the temperature of the substrate by placing the thermocouple directly on the substrate, but the high frequency electromagnetic wave (radio frequency; rf) from the plasma source causes noise and it hinders to obtain exact temperature.